1. Field of the Invention
The present invention relates to a recording medium useful for color recording, and particularly for ink-jet color recording. The present invention also relates to a recording method employing the recording medium.
2. Related Background Art
Coated paper which has an ink-receiving layer which contains a porous inorganic pigment formed on an ink-absorbent paper base has been used for recording mediums for ink-jet recording as described in Japanese Patent Application Laid-Open No. 56-148585. The porous inorganic pigment contained in the coating layer is exemplified by silica having superior color-developing properties as described in Japanese Patent Application Laid-open No. 56-185690, and by many other materials including calcium carbonate, alumina, and so forth.
The aforementioned coated paper is required to have the performance of providing images in high density and high sharpness with high resolution, and is further required to be responsive to high-speed print output.
In serial type color ink-jet printers, in order to achieve high-speed print output, increasing the driving-frequency of the head is essential, and also important is the capability of the head to conduct printing both in the forward movement direction and in the backward movement direction. A color ink-jet system is considered as an example in which four ink heads respectively for the colors of black (Bk), yellow (Y), magenta (M) and cyan (C) are arranged in the order of Bk, Y, M, and C from the back side to the front side along the forward direction of the head movement. Here, the direction of the movement of the ink heads starting from the home position is defined to be "forward" direction, and the reversed direction to be "backward" direction. The order of the colors of dots plotted with the color inks to provide colors of red (R), green (G), and blue (B) in the forward movement of the head is reversed in the backward movement. With the coated paper derived in the above cited prior art techniques, reversal of the dotting order of color inks results in change of color tone at mixed color portions, which hinders printing in back-and-forth directions.
The above prior art techniques have further disadvantages as mentioned below. For example, when the silica having a large specific surface area as disclosed in Japanese Patent Application Laid-Open No. 56-185690 is used for obtaining a sharp image with high density, the dye applied on a recording medium changes its color over time to deteriorate the recorded image even when it is stored in ordinary environmental conditions, like posting on an indoor wall. On the contrary, with a pigment such as calcium carbonate, kaolin, and talc having a small specific surface area, the above-mentioned indoor discoloration is retarded, however the derived image density is low without sharp image quality. Therefore, the suppression of the indoor discoloration is not consistent with high image density, and this inconsistency could not be removed by prior techniques.
The inventors of the present invention became aware of the fact that the indoor discoloration of the recorded image results only when coated paper is used, and does not result when non-coated paper such as ordinary PPC paper is used, and also that this indoor discoloration differs intrinsically from the dye discoloration caused by projection of UV light or visible light, and arises even in the absence of the light.
The inventors considered that the indoor discoloration results from oxidative decomposition of the dye which is caused by interaction of the dye, the pigment and an oxidative gas with each other. Accordingly, it may be assumed that a larger specific surface area of the used pigment causes more rapid oxidative decomposition reactions. From this assumption, the fact is understood that the higher degree of indoor discoloration is caused on coated paper having larger specific surface area of the pigment used.
On the other hand, the image density will be higher if the active surface for dye absorption is larger in the vicinity near the surface layer of the coat layer.
The inventors of the present invention already proposed a recording medium free from the aforementioned problems of indoor discoloration and insufficient image density, as shown in EP 405 417 A1, etc. This recording medium, however, does not satisfactorily solve the problem of color tone change in printing in the back and forth directions.